CN1068384C - Manganese-containing material with high tensile strength - Google Patents

Abstract

The invention relates to an iron-based powder for producing components by powder metallurgy for the briquetting and sintering of powders. The powder contains 0.25-2.0 wt% Mo, 1.2-3.5wt% Mn, 0.5-1.75wt% Si and 0.2-1.0wt% C and no more than 2% impurities, including less than 0.25wt% Cu. The invention also includes a method for making sintered parts and sintered products using such iron powder.

Description

Manganese-containing material with high tensile strength

The invention relates to an iron-based powder for the production of components by compaction and sintering. In particular, the invention relates to substantially nickel-free powder compositions which, when sintered, give components having valuable properties, such as high tensile strength. Such components can be used in the automotive industry. The invention also relates to powder metallurgically produced components of such powders and to a method of powder metallurgically producing such components.

In the field of powder metallurgy, nickel is a relatively common alloying element in iron-based powder compositions. Nickel is generally believed to improve the tensile strength of sintered parts prepared with iron powders containing up to 8% nickel. In addition, nickel promotes sintering, increases hardenability and at the same time has a favorable influence on elongation.

A powder currently on the market, the application of which gives a product with properties similar to those obtained with the product according to the invention, is Distaloy(R) AE, which contains 4% by weight of nickel.

But there is a growing demand for nickel-free powder, because nickel is expensive, dust is generated during powder processing, and a small amount of nickel can cause allergic reactions. From an environmental point of view, the use of nickel should be avoided.

It is therefore an object of the present invention to provide nickel-free compositions which, at least in some respects, have the same properties as nickel-containing compositions.

A second aim is to provide low cost, environmentally acceptable materials.

A third object is to provide sintered products having, after low and high temperature sintering, tensile strength values superior to those obtained with Distaloy(R) AE.

According to the present invention, metal powders containing 0.25-2.0wt% Mo, 1.2-3.5wt% Mn and 0.5-1.75wt% Si, 0.2-1.0wt% carbon and 2wt% impurities in addition to iron exhibit very interesting performance. Thus, when the metal powder according to the invention is compacted and then sintered at high temperature, tensile strengths of up to 1200 MPa can be obtained.

A preferred iron-based powder according to the invention contains 0.5-2 wt% Mo, 1.2-3.wt% Mn and 0.5-1.5 wt% Si, 0.3-0.9 wt% C and less than 2 wt% Contains less than 0.25% Cu. Said impurities may consist of Cr, Ni, Al, P, S, O, N, Be, B, etc. in addition to Cu. Their content is less than 0.5 wt%.

Mo can be used as metal powder, partially prealloyed with iron or prealloyed with iron. The hardenability of the compacted material increases when Mo is added to the iron powder, and it is recommended that the Mo content be at least 0.25% by weight. However, as the content of Mo increases, the compressibility decreases and thus the density decreases, and the amount of Mo should preferably be less than 2.0 wt%. In addition, a too high content of Mo, especially in combination with a high content of C, makes the sintered material hard and brittle, and the strength of the material decreases.

Mo is preferably added in the form of a pre-alloyed matrix powder to obtain a more uniform microstructure consisting of bainite and martensite in the sintered material.

According to a particularly preferred embodiment of the invention, Mo is added in the form of Astaloy Mo or Astaloy 85 Mo (from HSganös AB, Sweden), which contain 1.5 and 0.85 wt. % Mo, respectively.

Mn and Si can improve hardenability. Suitable addition amounts of these elements are above 1.2 and 0.5 wt %, respectively. However, too high contents of Mn and Si, such as above 3.5 and 1.75wt% respectively, lead to a decrease in compressibility and can cause oxidation problems. A high content of Mn and Si in the prealloyed base powder has a strong solution hardening effect, while these elements added in elemental form have a strong affinity for oxygen.

However, if these elements are added in the form of master alloys, their affinity for oxygen is reduced and their susceptibility to oxidation is reduced. Therefore, according to a preferred embodiment of the present invention, Mn and Si are added in the form of Fe-Mn-Si master alloy, and the composition of said master alloy is Si of 10～30wt%, Mn of 20～70wt%, the rest It is Fe, and the weight ratio of Mn/Si is between 1 and 3. Such a master alloy consisting essentially of (Fe,Mn) ₃ Si and (Fe,Mn) ₅ Si ₃ is proposed in EP97 737 . Since the Fe-Mn-Si master alloy forms a transitional liquid phase during the sintering process, accelerates sintering, promotes diffusion, increases the amount of martensite and makes pores round, so the master alloy can improve compressibility , the microstructure of the sintered material becomes more uniform. Using this master alloy, it is possible to avoid the large shrinkage caused by silicon, resulting in close to zero dimensional changes. In addition, Mn and Si may be added in the form of iron-manganese and iron-silicon.

If the amount of C, usually added in the form of graphite powder, is less than 0.2%, the tensile strength will be too low, and if the amount of C is above 1.0%, the sintered part will be too brittle. Parts prepared with compositions according to the invention, wherein the C content is low, show good ductility, acceptable tensile strength, while articles prepared with compositions with higher C content have lower ductility and greater tensile strength. Graphite addition must be determined according to the sintering atmosphere. The more hydrogen in the atmosphere, the more graphite needs to be added due to greater decarburization. Since some carbon is lost during sintering, the carbon content of the sintered product is slightly lower than that of the iron-based powder. Therefore, the carbon content of the sintered product is usually between 0.15-0.70 wt%.

As possible impurities, Ni, Cu and Cr may be mentioned. These elements may be present in amounts of less than 0.25% by weight each, but are preferably present in trace amounts, ie up to 0.1% by weight of the composition. Other possible impurities are Al, P, S, O, N, Be, B, the amounts of which are indicated in the claims. The total content of impurities should be less than 2 wt%, however, preferably less than 1 wt%.

The effect of adding different amounts of Mo, Mn/Si and C is presented in Figures 1, 2 and 3, respectively.

In addition to iron-based powders, the invention also relates to methods of producing components with these new powders and to the components produced. Said powder metallurgy process is carried out in a conventional manner known to those skilled in the art, including the steps of compaction, sintering and optional repressurization and sintering and/or quenching and tempering of the powder. The compaction step can be performed by cold pressing and warm pressing, and the sintering step can be performed by low temperature sintering as well as high temperature sintering. The sintering atmosphere and sintering time impart properties to the final product known in the art.

In this regard, it may be mentioned that WO 80/01083 proposes alloy steel articles having a composition similar to the present product. However, these known products are conventional, cast non-porous products produced by casting. In order to obtain an apparently complete bainite structure, specific heat treatment and isothermal tempering are subsequently carried out. Apart from the range of alloying elements, these known products differ from the products prepared according to the present invention in several respects, such as the type of starting material, process and microstructure.

It has surprisingly been found that by using novel iron-based mixtures it is possible to obtain materials with tensile strengths up to about 1200 MPa. These remarkably high values can be obtained, for example, by sintering at a high temperature between about 1200-1280° C. for about 1 hour in a hydrogen atmosphere. It is worth noting that the pressed body made of the iron-based powder according to the present invention can also be sintered at a temperature between 1110 and 1150 ° C by low temperature sintering, and can also be sintered by a very high tensile strength of up to 1000 MPa. distinguish. It can also be observed that at relatively moderate densities, such as 6.8-7.0 g/cm ³ , unexpectedly high strengths are obtained. Furthermore, it has been found that the new composition exhibits good stability with respect to dimensional changes at different densities.

In short, the high tensile strength of the sintered product according to the invention combined with the low cost of the powder and the modest impact on the environment make the invention particularly advantageous.

The invention is described in more detail in the following examples.

Example

Different alloy compositions have been tested for Mo, Mn, Si and C. A master alloy with a composition of 45% Mn, 21% Si and the balance Fe was used in the following tests. Said master alloy, graphite and in some tests Mo powder were mixed with ASC100.29, Astaloy 85 Mo or AstaloyMo. The tensile sample was pressed at 600MPa, and then sintered at 1250°C for 30-60 minutes in a mixed atmosphere of hydrogen and nitrogen. The best strength properties are obtained when the molybdenum content is 0.25-2.0%, see Figure 1. When the amount of molybdenum added is small, the hardenability is too low, and when the amount of molybdenum added is too high, the density is too low. The molybdenum content is preferably between 0.5-2%. The powders tested contained 2.8% Mn, 1.2% Si, 0.7% graphite in addition to iron and varying amounts of Mo.

Less master alloy addition results in lower hardenability of the material, resulting in lower strength. High alloy additions result in bulky master alloys, lower compressibility, and can lead to increased expansion of the material. Reduced strength due to low density. The optimum additions of manganese and silicon are 1-3.5% Mn and 0.5-1.75% Si respectively, see Figure 2. The powders tested included 0.85% Mo and 0.7% graphite in addition to iron and varying amounts of Mn, Si.

The carbon content analyzed depends on the amount of graphite added and also on the sintering atmosphere used. The higher the hydrogen content used, the greater the decarburization effect. The optimum carbon content of the sintered sample is between 0.15% and 0.7%, see Figure 4. In these tests, this corresponds to 0.3-0.9% graphite in the powder composition, see FIG. 3 . The iron-based powders tested contained 0.85% Mo, 1.8% Mn, 0.8% Si and varying amounts of graphite.

Increase the strength of the material by increasing the sintering temperature and increasing the sintering time. This is mainly due to better diffusion of the alloying elements mixed, which improves the hardenability and thus the strength of the material. This effect can be seen in Figure 5 for a powder consisting of Fe, 0.85% Mo, 1.8% Mn, 0.8% Si and 0.5-0.7% graphite.

For the newly developed material, the change in size is stable at different densities. This is advantageous when producing parts with a high inherent density variation. Keeping tight tolerances is made easier by using dimensionally stable materials. Figure 6 presents the dimensional change for Fe-0.85Mo-1.8Mn-0.8Si-(0.6-0.7C) pressed at 400, 600 and 800 MPa. Sintering was performed at 1120°C and 1250°C. The variation of size is 0.03% and 0.12%, respectively, and the variation range of density is 6.6-7.1 g/cm ³ .

Claims (9)

1. An iron-based powder for the production of porous parts by pressing and sintering of the powder, comprising, in addition to iron: 0.25-2.0wt% Mo 1.2-3.5wt% Mn 0.5-1.75wt% Si 0.2-1.0wt% C and no more than 2wt% impurities, including less than 0.25wt% Cu. 2. A powder according to claim 1, wherein the amount of Mo is 0.5-2.0 wt%. 3. A powder according to claim 1 or 2, wherein the amount of Mn is 1.2-3.0 wt%. 4. A powder according to claim 1, wherein the amount of Si is 0.5-1.50 wt%. 5. A powder according to claim 1, wherein the amount of C is 0.3-0.9 wt%. 6. A powder according to claim 1, wherein Mn and Si are added in the form of ferromanganese, ferrosilicon or silicon-manganese-iron master alloy. 7. 6. A powder according to claim 6, wherein the manganese/silicon weight ratio of said silicon-manganese-iron master alloy varies between 1-3. 8. A powder according to claim 1 or 2, wherein Mo is added in the form of a prealloy of Fe and Mo. 9. A powder according to claim 1, characterized in that it includes unavoidable impurities in the following ranges in weight percent: Cr＜0.25 Cu＜0.25 Ni<0.25 Al<0.20 P<0.05 S<0.05 O<0.03 N<0.02 Be<0.01 B<0.02 Other <0.5.

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